1. Field of the Invention
The present invention relates to a light-emitting device and a lighting device which utilize electroluminescence.
2. Description of the Related Art
In recent years, research and development have been extensively conducted on light-emitting elements utilizing electroluminescence (EL). In a basic structure of such a light-emitting element, a layer containing a light-emitting substance (an EL layer) is interposed between a pair of electrodes. By voltage application to this element, light emission can be obtained from the light-emitting substance.
Since such a light-emitting element is of self-light-emitting type, it is considered that the light-emitting element has advantages over a liquid crystal display, such as high pixel visibility and the eliminated need for a backlight. Accordingly, such a light-emitting element is thought to be suitable as a flat panel display element. Such a light-emitting element is also highly advantageous in that it can be thin and lightweight. Further, very high speed response is also one of the features of such an element.
Furthermore, since such a light-emitting element can be formed into a film form, planar light emission can be easily obtained. Therefore, a large-area element using planar light emission can be formed. This is a feature which is difficult to obtain with point light sources typified by incandescent lamps and LEDs or linear light sources typified by fluorescent lamps. Thus, the light-emitting element also has great potential as a planar light source applicable to a lighting device or the like.
In order to apply such a light-emitting element to a light-emitting device, the light-extraction efficiency of the light-emitting element needs to be improved. As a method of improving the light-extraction efficiency of a light-emitting element, a structure in which a micro optical resonator (a microcavity) utilizing a resonant effect of light between a pair of electrodes is used and regions having different cavity lengths are provided to improve the viewing angle dependence of a light-emitting element (e.g., see Patent Document 1), and the like have been proposed.